1. Field of the Invention
The present invention concerns crosslaminates.
More particularly, the present invention concerns laminates of films of which at least two are uniaxially or unbalanced biaxially oriented, and in which the main direction of orientation in one of these films crosses the main direction in the other one.
2. Description of the Related Art
Crosslaminates of oriented films from synthetic polymer materials have been commercially produced since 1968, then mainly as described in GB-A-0792976 (Rasmussen) of May 23, 1955. To the inventor's knowledge the total annual worldwide production today amounts to about 30,000 tons. The crosslaminate is used in particular as industrial bags, cover sheet, tarpaulins, pond-liners and similar products.
Compared to generally unoriented films, crosslaminates exhibit very improved strength properties, seen in relation to the square meter weight, and since the raw material price is the most important part of the costprice, the crosslamination technology can serve to reduce the cost by reduction of weight. Compared to biaxially oriented film, crosslaminates made (under adequate conditions) from similar polymers, exhibit dramatically improved tear propagation resistance.
Nevertheless, as the figure 30,000 tons annual production indicates, the success of crosslamination technology in the marketplace has been limited. An important reason for this is difficulties in maintaining a high tear propagation resistance and at the same time adequate bonding strength in relatively thin laminates, while the main advantages in particular should be the possibility to reduce the weight. The high tear propagation resistance in adequately produced crosslaminates is based on local delamination around the location where the tear propagates. Due to the unbalanced orientation in the individual films and criss-crossing of the main directions of the orientation, one film will then have a tendency to propagate the tear in one direction and another film will tend to propagate the tear in another direction. Thereby there will be a tendency to eliminate the bonding at the location where the forces are concentrated, and if this tendency is sufficiently pronounced, the tear will “fork out” under a local delamination, and the “notch effect” of the tearing will almost be eliminated. Hereby there will be, generally speaking, “competition” between the adhesive forces which try to withstand delamination, and the cohesive forces in each film which try to avoid a rupture or flow along any direction which is not parallel with the main direction of orientation. The said adhesive forces are (still generally speaking) independent of the thickness of the films, while the said cohesive forces are mainly proportional to the film thickness, when all other parameters are unchanged. As a consequence of this “competition”, “thin” crosslaminates will either exhibit a relative poor tear propagation resistance or a relatively high tendency to delamination. This is much less of a problem for crosslaminates of “thick” layers. For industrial bags of gauge higher than about 60-70 grams per square meter this “competition” will usually not cause serious problems since filled bags are usually not subjected to delaminating forces, which means that a low bonding strength can be chosen, but the matter is very important e.g. for tarpaulins, cover sheets and similar products which will be subjected to repeated flexing during use, e.g. will flap in the wind. As a matter of practical experience the inventor and his licensees have found that in a tarpaulin made from a two-ply crosslaminate based on combinations of LLDPE- and HMWHDPE types, each of the films must be of a gauge of at least 45-50 gm−2, otherwise either the bonding strength or the tear propagation resistance will be unacceptable to the users. These experiences concern tarpaulins for “static” uses where there will not occur much flapping in the wind. For “dynamic” uses such as cover over trucks or goods wagons, where the tarpaulin will be subjected to strong, repeated flapping, the gauge required is much higher. In low gauge film (e.g. of gauge between about 15-50 gm−2 destined for the final consumer, there is also a need for a high tear propagation resistance combined with good bonding, since such tear strength is a property which the consumer easily can evaluate. Examples of such films are films for wrapping of consumer goods and the different kinds of household films.
Several methods have been suggested for achieving an adequate combination of bonding strength and tear propagation resistance in crosslaminated films. They are all described in WO03/074264 and all relate to a combination of a strong or relatively strong bonding in spots or lines, and weak bonding or no bonding over the rest of the film area. Of this known art, only the method and structure claimed in the said WO03/074264 has had industrial importance.